2. Add Two Numbers

class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        ListNode preNode = new ListNode(0);        ListNode tail = preNode;        int pre = 0;        while (l1!=null&&l2!=null){            int val1 = l1.val;            int val2 = l2.val;            int res = val1+val2+pre;            pre = res/10;            tail.next = new ListNode(res%10);            tail=tail.next;            l1=l1.next;            l2=l2.next;        }        while (l1!=null){            tail.next = new ListNode((l1.val+pre)%10);            tail=tail.next;            pre = (l1.val+pre)/10;            l1=l1.next;        }        while (l2!=null){            tail.next = new ListNode((l2.val+pre)%10);            tail=tail.next;             pre = (l2.val+pre)/10;            l2= l2.next;        }        if (pre!=0)tail.next = new ListNode(pre);        return preNode.next;    }}